Piezoelectric thin film element

ABSTRACT

A piezoelectric thin film element includes a bottom electrode, a piezoelectric layer, and a top electrode on a substrate. The piezoelectric layer includes, as a main phase, a perovskite-type oxide. The bottom electrode has a surface roughness of not more than 0.86 nm in arithmetic mean roughness Ra or not more than 1.1 nm in root mean square roughness Rms. The bottom electrode has a (111) preferential orientation in a direction perpendicular to the substrate.

The present application is a Continuation Application of U.S. patentapplication Ser. No. 12/588,484, filed on Oct. 16, 2009, which is basedon and claims priority from Japanese Patent Application Nos. 2008-311972and 2009-114122 filed on Dec. 8, 2008 and May 11, 2009, respectively,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a piezoelectric thin film element using apiezoelectric layer formed of lithium potassium sodium niobate etc.

2. Related Art

A piezoelectric material is processed into various piezoelectricelements in accordance with different purposes, particularly, it iswidely used for functional electronic components such as an actuator forgenerating deformation by applying voltage or a sensor for generatingvoltage from the deformation of elements in a reverse way, etc.

As a piezoelectric material used for an actuator or a sensor, alead-based dielectric material having large piezoelectriccharacteristics, especially, Pb(Zr_(1-x)—Ti_(x))O₃-based perovskite-typeferroelectric called PZT, has been widely used thus far, and thepiezoelectric material is formed by sintering oxide which is generallyformed of individual elements.

In addition, in recent years, it is desired to develop a piezoelectricmaterial not containing lead from environmental consideration, and thus,lithium potassium sodium niobate (general formula:(Na_(x)K_(y)Li_(z))NbO₃ (0<x<1, 0<y<1, 0<z<1, x+y+z=1), etc., has beendeveloped. Since the lithium potassium sodium niobate has piezoelectriccharacteristics comparable to PZT, it is expected as a potentialcandidate for a lead-free piezoelectric material.

On the other hand, currently, downsizing and high performance arestrongly demanded also in the piezoelectric element as downsizing andhigh performance in various electronic components progress. However, ina piezoelectric element produced by a manufacturing method mainly by asintering process which is a conventional manufacturing method,especially when a thickness thereof becomes 10 μm or less, a dimensionthereof comes close to that of a crystal grain composing a material, andinfluence cannot be ignored. Therefore, there arises a problem thatvariation or deterioration in characteristics becomes evident, and amethod for forming a piezoelectric material by employing a thin-filmtechnique etc., as an alternative to the sintering process is beingresearched in recent years in order to avoid such a problem.

Recently, a PZT thin film formed by a RF sputtering method has been putto practical use as an actuator for a high definition inkjet printerhead or a compact and low cost gyro sensor (e.g., see JP-A 10-286953 andnon-patent literary document of Kiyoshi Nakamura “High performance ofpiezoelectric material and advanced applied technologies” (in 2007)published by Science & Technology Co., Ltd). In addition, it has beenproposed a piezoelectric thin film element using a lithium potassiumsodium niobate not using lead (e.g., see JP-A 2007-19302).

It is possible to manufacture a inkjet printer head or a gyro sensorwith small environmental burden by forming a lead-free piezoelectricthin film as a piezoelectric thin film. A basic research in thinning oflithium potassium sodium niobate is progressing as a concrete candidatefor the lead-free piezoelectric thin film. However, it is difficult toreproducibly manufacture a piezoelectric thin film of lithium potassiumsodium niobate having a large piezoelectric constant equivalent to thatof PZT.

In addition, in order to reduce the cost in applications, it isessential to establish a technique to controllably form a lead-freepiezoelectric thin film on a Si substrate or a glass substrate. When anactuator or a sensor is manufactured using a Si substrate or a glasssubstrate, it is required to provide electrodes on and under thepiezoelectric thin film, however, since surface roughness of top/bottomelectrodes and the piezoelectric thin film are rough and surfaceirregularities are large in the prior art, it is difficult to stablyprocess into a predetermined shape or a surface state when a device isformed by applying a process, thus, there is a problem that a decreasein manufacturing yield occurs. In addition, since the irregularities ondevice surfaces are large, deterioration in aging characteristics occursdue to generation of leakage current or electric field intensity of agrain boundary portion of a piezoelectric material, and as a result, adecrease in piezoelectric constant or product lifetime occurs.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a piezoelectric thin filmelement excellent in piezoelectric characteristics.

(1) According to one embodiment of the invention, a piezoelectric thinfilm element comprises a bottom electrode, a piezoelectric layer and atop electrode on a substrate,

wherein the piezoelectric layer comprises as a main phase aperovskite-type oxide represented by (Na_(x)K_(y)Li_(z))NbO₃ (0≦x≦1,0≦y≦1, 0≦z≦0.2, x+y+z=1), and

the bottom electrode comprises a surface roughness of not more than 0.86nm in arithmetic mean roughness Ra or not more than 1.1 nm in root meansquare roughness Rms.

In the above embodiment (1), the following modifications and changes canbe made.

(i) The bottom electrode comprises a (111) preferential orientation in adirection perpendicular to the substrate.

(ii) The bottom electrode comprises a bonding layer on the substrate andan electrode layer on the bonding layer, and the bonding layer comprisesa Ti film of not less than 0.084 nm and not more than 10 nm inthickness.

(iii) The piezoelectric layer formed on the bottom electrode comprises amaximum-minimum surface roughness of not more than 23% with respect toan average film thickness of the piezoelectric layer.

(iv) The piezoelectric layer formed on the bottom electrode comprises anarithmetic mean surface roughness Ra or root mean square surfaceroughness Rms of not more than 2.7% with respect to an average filmthickness of the piezoelectric layer.

(v) The piezoelectric layer comprises a surface roughness of not morethan 4.1 nm in arithmetic mean roughness Ra, not more than 4.8 nm inroot mean square roughness Rms, or not more than 137.7 nm inmaximum-minimum roughness.

(vi) The piezoelectric layer comprises a part of an ABO₃ crystal layer,an ABO₃ amorphous layer, or a mixed layer comprising a crystal and anamorphia of ABO₃, where A comprises at least one element selected fromLi, Na, K, Pb, La, Sr, Nd, Ba and Bi, B comprises at lest one elementselected from Zr, Ti, Mn, Mg, Nb, Sn, Sb, Ta and In, and O is oxygen.

(vii) The piezoelectric layer comprises a (001) preferential orientationin a direction perpendicular to the substrate.

POINTS OF THE INVENTION

A bottom electrode in crystalline state is oriented to a (111) plane asa closest-packed plane of atoms when its crystal structure is a cubiccrystal as in Pt etc. A lithium potassium sodium niobate film formed onthe Pt bottom electrode in crystalline state is therefore oriented to a(001) plane based on the structure of a crystal surface exposed to thesurface of the Pt bottom electrode, so that it can be significantlyimproved in surface flatness. As a result, a piezoelectric thin filmelement can be excellent in piezoelectric characteristics since thesurface flatness can be improved by controlling the surface roughness ofthe bottom electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Next, the present invention will be explained in more detail inconjunction with appended drawings, wherein:

FIG. 1 is a schematic cross view showing a substrate with apiezoelectric thin film in Example 1 of the invention;

FIG. 2 is a view showing an X-ray diffraction pattern of the substratewith the piezoelectric thin film in Example 1;

FIGS. 3A and 3B are AFM observation images showing surfaces of Pt bottomelectrodes in Example 1 and Comparative Example 1 observed by an AFM;

FIGS. 4A and 4B are AFM observation images showing surfaces of KNNpiezoelectric thin films in Example 1 and Comparative Example 1 observedby an AFM;

FIG. 5 is a schematic cross view showing a piezoelectric thin filmelement in Example 2 of the invention;

FIG. 6 is a schematic cross view showing a piezoelectric thin filmelement in Example 3 of the invention;

FIG. 7 is a correlation diagram between applied electric field and apiezoelectric constant of the piezoelectric thin film element;

FIGS. 8A and 8B are correlation diagrams between surface roughness and apiezoelectric constant of the Pt bottom electrode;

FIG. 8C is a correlation diagram between surface roughness and apiezoelectric constant of the Pt bottom electrode;

FIGS. 9A and 9B are correlation diagrams between surface roughness ofthe Pt bottom electrode and surface roughness of the KNN piezoelectriclayer formed on an upper portion thereof;

FIGS. 10A and 10B are correlation diagrams between surface roughness anda piezoelectric constant of a surface of the piezoelectric layer;

FIGS. 11A and 11B are correlation diagrams between a relative value ofthe surface roughness with respect to an average film thickness and apiezoelectric constant of the piezoelectric layer; and

FIG. 12 is a correlation diagram between a relative value ofmaximum-minimum surface roughness with respect to an average filmthickness and a piezoelectric constant of the piezoelectric layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the piezoelectric thin film element of the inventionwill be explained as follows.

The piezoelectric thin film element in the present embodiment has asubstrate, an oxide film formed on a surface of the substrate, a bottomelectrode formed on the oxide film, a perovskite-type piezoelectriclayer formed on the bottom electrode, and a top electrode formed on thepiezoelectric layer.

A main phase of the piezoelectric layer is a perovskite-type oxiderepresented by (Na_(x)K_(y)Li_(z))NbO₃ (0≦x≦1, 0≦y≦1, 0≦z≦0.2, x+y+z=1)and the substrate is a Si substrate.

Furthermore, surface roughness of the bottom electrode is either 0.86 nmor less in arithmetic mean roughness Ra or 1.1 nm or less in root meansquare roughness Rms.

A Si substrate is preferable as the above-mentioned substrate because ofits low cost and industrial accomplishments, however, it is possible touse a MgO substrate, a SrTiO₃ substrate, a SrRuO₃ substrate, a glasssubstrate, a silica substrate, a GaAs substrate, a GaN substrate, asapphire substrate, a Ge substrate or a stainless substrate, etc.,besides the Si substrate.

The oxide film formed on the surface of the Si substrate includes athermal oxide film formed by thermal oxidation and a Si oxide filmformed by a CVD (Chemical Vapor Deposition) method, etc. Alternatively,for an oxide substrate such as silica glass, MgO, SrTiO₃ or SrRuO₃,etc., a bottom electrode such as a Pt electrode may be directly formedwithout forming the oxide film on the substrate.

The bottom electrode is preferably an electrode layer formed of Pt or analloy consisting of mainly Pt, or, an electrode layer having a laminatedstructure thereof. In addition, it is preferable that the bottomelectrode is formed preferentially orienting to a (111) plane directionin a direction perpendicular to the substrate. A bonding layer (adhesionlayer) such as Ti, etc., for enhancing adhesion with the substrate maybe provided between the substrate and the electrode layer formed of Ptor an alloy consisting of mainly Pt.

On the surface of the bottom electrode such as Pt, etc., which ispreferentially oriented to a desired direction such as a (111) plane,etc., a shape for accelerating smoothing is realized. As a result ofmeasuring surface roughness of the Pt bottom electrode by an AtomicForce Microscopy (AFM), at least one or more of any of a portion of 0.86nm or less in arithmetic mean roughness, a portion of 1.1 nm or less inroot mean square roughness and a portion of 11.5 nm or less inmaximum-minimum roughness are present in an observation range of 1 μmsquare. Alternatively, surface irregularities may be measured by a SEM(scanning electron microscope) or a TEM (transmission electronmicroscope).

The piezoelectric layer may be a piezoelectric layer of which main phaseis a perovskite-type oxide represented by (Na_(x)K_(y)Li_(z))NbO₃(0≦x≦1, 0≦y≦1, 0≦z≦0.2, x+y+z=1) and, for example, a predeterminedamount of Ta (tantalum) or V (vanadium) may be doped to potassium sodiumniobate or lithium potassium sodium niobate within a range not losingthe piezoelectric characteristics or the surface roughness. Thepiezoelectric layer is formed using the RF sputtering method.

In the meantime, when the bottom electrode is formed on the substratesuch as a Si substrate without considering about lattice match with alithium potassium sodium niobate film (including a case of a potassiumsodium niobate film) and the lithium potassium sodium niobate film isformed on an upper portion thereof, a polycrystalline thin film isformed. Since it is conventionally formed without taking intoconsideration of crystal orientation, the lithium potassium sodiumniobate film is formed as a random-oriented polycrystalline thin film.

In addition, the film is conventionally formed without considering abouta size of surface irregularities of the bottom electrode which is a baseof the piezoelectric thin film. Therefore, the surface irregularities ofthe lithium potassium sodium niobate film are very large. The reasonthereof is considered that plane direction dependence of a crystalgrowth rate affects the formation of the piezoelectric thin film andthat a growth direction of the piezoelectric thin film formed on theupper portion of the bottom electrode becomes random due to the largeirregularities on the surface of the bottom electrode.

Therefore, firstly, it was examined to manufacture a polycrystallinethin film having uniaxial orientation so that a surface of the lithiumpotassium sodium niobate film faces the same plane direction. As amethod thereof, on the upper portion of the bottom electrode as a basewhich is not in amorphous state but in a crystalline state, the lithiumpotassium sodium niobate film is formed.

The bottom electrode in the crystalline state is oriented to a (111)plane as a closest-packed plane of atoms when its crystal structure is acubic crystal as in Pt etc. For example, in case of forming a film of Pton a substrate at a room temperature, a Pt thin film in the amorphousstate is formed, however, in case of forming a film by heating thesubstrate using a sputtering method, a Pt thin film in the crystallinestate preferentially orienting to (111) is formed. The lithium potassiumsodium niobate film formed on the Pt bottom electrode in the crystallinestate becomes a state oriented to a (001) plane based on a structure ofa crystal plane exposed to the surface of the Pt bottom electrode, whichresults in a significant improvement in surface flatness.

Next, it was examined to planarize the surface of the bottom electrodeto be a base such as Pt, etc., so that the lithium potassium sodiumniobate film grows in one direction. One of a method for planarizing thesurface of the bottom electrode such as Pt, etc., is a method in whichthe surface irregularities of a bottom electrode layer is decreased bystrictly controlling a film thickness of the bottom electrode.

In addition, another method for planarizing the surface of the bottomelectrode such as Pt, etc., is a method in which a bottom electrodelayer such as polycrystalline Pt, etc., is formed by controlling crystalgrain size thereof to be uniform, thereby planarizing the surface of thebottom electrode layer.

As a result of planarizing the surface of the bottom electrode layer,surface flatness is significantly improved in the lithium potassiumsodium niobate film formed on the upper portion of the bottom electrode.

A method for decreasing the surface irregularities of a bottom electrodelayer by strictly controlling a film thickness of the bottom electrodeis as follows.

In case of a sputtering film formation method, a film formation rate isincreased or decreased by changing an input power, the film thickness isincreased or decreased in the film formation for a certain time, and itis thereby possible to control the film thickness. In addition, the sameeffect is obtained by increasing and decreasing pressure of Ar which isa film formation action gas used for sputtering. When the film thicknessof the polycrystalline film such as Pt, etc., is increased (when growthtime is extended), many crystal grains which are abnormally grown arescattered in the film, furthermore, crystal grain size thereof isincreased. As a result, since large and small crystal grains existtogether in spots, a concavo-convex shape on the surface of the Ptbecomes large. In order to realize a bottom electrode in a state thatthe abnormally grown crystal grain does not exist locally at all or in astate that the size of the abnormally grown crystal grain is small, itis possible to planarize by strictly controlling the film thickness (inthis case, by decreasing the film thickness).

In addition, the bottom electrode such as Pt, etc., is intermittentlyformed in multiple stages, i.e., formed as a multilayer film of a thinfilm. As a result, an intermittent film formation, in which the filmformation is suspended once before the abnormal growth of the Pt crystalgrains becomes significant and a film is substantially formed again, isrepeated for gradually laminating, and it is thereby possible to ensureuniformity of the film thickness. Furthermore, by rotating and revolvingthe substrate on which the bottom electrode is formed during the filmformation, it is possible to decrease distribution of the film thicknessof Pt, etc., and to control a film thickness distribution.Alternatively, in case of forming a film by the sputtering method, araw-material target side may be rotated and revolved. Also, it ispossible to control the film thickness by carrying out a machiningprocess such as grinding or polishing, a chemical mechanical polishingcalled CMP, or sputtering etching by Ar ions, etc., with respect to thesurface of the bottom electrode having a nonuniform film thickness.

In addition, a method for planarizing the surface of the bottomelectrode by forming a bottom electrode such as polycrystalline Pt,etc., by strictly controlling crystal grain size thereof to be uniformis as follows.

By the film formation that orientations of each crystal grain arealigned so that the thin film of Pt, etc., is highly oriented to a (111)plane direction, it is possible to suppress influences such as localabnormal growth of the crystal grain or extension of inter-grainboundaries caused by growth of the crystal grain in random directions.As a result, the crystal grain size becomes uniform and the surface ofthe bottom electrode is planarized. Similarly to the method forachieving high orientation, it is effective to form the Ti bonding layer(adhesion layer) to be a base in a film thickness of 10 nm or less forcontrolling uniformity of the crystal grain size, preferably, in a filmthickness of 2.1 nm or less. It is desirable to form a thin bondinglayer of at least about 1 atomic layer for high planarization, thus, alower limit of the Ti bonding layer is determined to be 0.084 nm.Furthermore, it is efficient to set a Pt film formation temperature to200° C. or more which is higher than a room temperature. In addition, aseveral nm thick seed layer formed of Pt or an alloy compositioncontaining Pt which is a core of each Pt crystal grain is formed as abase of the bottom electrode, the bottom electrode is formed thereon,and it is thereby possible to grow uniform Pt crystal grains.

In order to improve an oriented state of the lithium potassium sodiumniobate film, in the above-mentioned embodiment, an orientation controllayer for improving the orientation of the piezoelectric layer may beprovided between the bottom electrode and the piezoelectric layer.

It is preferable that the orientation control layer is LaNiO₃, LaAlO₃,SrTiO₃, SrRuO₃, La_(0.6)Sr_(0.4)FeO₃, La_(0.6)Sr_(0.4)CoO₃, KNbO₃,NaNbO₃ and a solid solution thereof, or a laminated body containing anyof them. For example, when the orientation control layer which is likelyto align on (001) such as LaNiO₃ is formed and the lithium potassiumsodium niobate thin film is formed on the upper portion thereof, itbecomes easy to obtain the lithium potassium sodium niobate thin filmpreferentially oriented to (001).

It is possible to manufacture a piezoelectric device such as variousactuators or sensors, or a compact apparatus, e.g., a MEMS (microelectro mechanical system) by forming the piezoelectric thin filmelement in the above-mentioned embodiment in a predetermined shape andproviding voltage application means or voltage detecting means. By theplanarization of the bottom electrode or a piezoelectric thin film, itis possible to realize improvement or stabilization of piezoelectriccharacteristics of the piezoelectric thin film element or thepiezoelectric device using the piezoelectric thin film element and alsoto improve manufacturing yield, and it is thereby possible to provide ahigh performance micro device at low cost.

EXAMPLES

Examples of the invention and Comparative Examples will be explainedbelow.

Example 1

FIG. 1 is a schematic cross sectional view showing a substrate with apiezoelectric thin film in Example 1. In Example 1, a substrate with apiezoelectric thin film was manufactured in which a bonding layer 2 wasformed on a Si substrate 1 having an oxide film, and a bottom electrodelayer 3 and a piezoelectric layer 4 of potassium sodium niobate (hereinreferred to as “KNN”) having a perovskite structure are formed on thebonding layer 2. A manufacturing method thereof will be explained below.

Firstly, a thermal oxide film (illustration omitted) was formed on asurface of the Si substrate 1, then, a bottom electrode was formed onthe thermal oxide film. In the present Example, the bottom electrode iscomposed of a 2.0 nm thick Ti film formed as the bonding layer 2 and a200 nm thick Pt thin film formed as the bottom electrode layer 3 on theTi film. The sputtering method was used for forming the Ti film and thePt thin film. For forming the Pt thin film, a Pt metal target was usedas a sputtering target and a 100% Ar gas was used for a sputtering gas.At the time of the film formation, sputtering input power was 75 W, asubstrate temperature was set to 300° C. and a film was formed 200 nmthick, thereby forming a Pt thin film which is a polycrystalline thinfilm of Example 1. In addition, as Comparative Example 1, a Pt thin filmwas made under the completely same film formation condition except thatthe substrate heating temperature was changed to 700° C.

The surface irregularities of the Pt thin film as the bottom electrodelayer 3 was examined by an AFM (Atomic Force Microscopy). For themeasurement, the AFM with a probe tip having a curvature radius of 1 nmwas used, and the surface roughness was analyzed by scanning at 4 nmpitch in a range of 1 μm×1 μm. As for the arithmetic mean roughness Ra,the root mean square roughness Rms and the maximum-minimum roughness(which corresponds to maximum height defined by JIS B0601) of thesurface roughness here, the surface roughness were estimated byfollowing method against the measured surface concavo-convex shape (acurved surface) (it is the same for the below described KNN film). Thesurface roughness are shown as a standard deviation of a maximum valueand a minimum value of the entire data of the irregularities measured bythe AFM.

The following formula (1) is a definitional equation of the root meansquare surface roughness Rms (the length unit is nm). In addition, thefollowing formula (2) is a definitional equation of the threedimensional arithmetic mean surface roughness Ra (the length unit is nm)against a center plane (volume formed by the plane surface and thesurface shape is equal above and below the plane. The detail isdescribed in “Large-sized Sample SPM Observation System Operation Guide”(April, 1996) published by Toyo Corporation.

$\begin{matrix}{{{Formula}\mspace{14mu} 1}\mspace{635mu}} & \; \\{{Rms} = \sqrt{\frac{\sum\limits_{i = 1}^{N}\left( {z_{i} - {z_{i}({ave})}} \right)^{2}}{N}}} & (1)\end{matrix}$

Here, N is the number of data measurement points, zi is a height of themeasurement point i, zi(ave) is an average value of zi, and Rms is rootmean square surface roughness.

$\begin{matrix}{{{Formula}\mspace{14mu} 2}\mspace{635mu}} & \; \\{{Ra} = {\frac{1}{LxLy}{\int_{0}^{Lx}{\int_{0}^{Ly}{{f\left( {x,y} \right)}{x}{y}}}}}} & (2)\end{matrix}$

Here, Lx is a dimension of a surface in an x direction, Ly is adimension of the surface in a y direction, f(x,y) is a roughness curvedsurface with respect to the center plane, and Ra is arithmetic meansurface roughness.

FIG. 3B is an AFM observation image showing a surface of the Pt bottomelectrode layer 3 in Example 1. In the Pt bottom electrode layer inExample 1, the arithmetic mean roughness Ra, the root mean squareroughness Rms and the maximum-minimum roughness Rmax, which are analyticvalues of the surface shape thereof, are respectively 0.83 nm, 1.1 nmand 11.5 nm. In addition, FIG. 3A is an AFM observation image showing asurface of a Pt bottom electrode layer in Comparative Example 1. In thePt bottom electrode layer in Comparative Example 1, the surfaceroughness is larger than that in Example 1, and the arithmetic meanroughness Ra, the root mean square roughness Rms and the maximum-minimumroughness of the surface were respectively 2.0 nm, 2.5 nm and 16.7 nm.Alternatively, the surface irregularities can be further reduced bydecreasing the film thickness of the Pt thin film to 200 nm or less.

Next, KNN thin films as the piezoelectric layer 4 were formed on a Sisubstrate with a Pt electrode in Example 1 and Comparative Example 1.The sputtering method was used also for forming the KNN thin film. Forforming the KNN thin film, the substrate was heated at 600° C. and thesputtering was performed by plasma using a mixed gas of Ar+O₂ (a mixtureratio of Ar:O₂=9:1). A sintered body target of (Na_(x)K_(y)Li_(z))NbO₃(x=0.5, y=0.5 and z=0) was used for a target. The film formation wasperformed at 100 W of RF output until the film thickness becomes 3 μm.

As a result of examining a crystal structure of the thus formed KNN filmby an X-ray diffractometer, it was found that the Pt thin film inExample 1 formed by heating the substrate at 300° C. was oriented to aPt (111) plane in a direction perpendicular to the surface of the Sisubstrate 1, as shown in the X-ray diffraction pattern of FIG. 2 (scanmeasurement of 2θ/θ). In addition, it was found that the KNN film formedon the Pt film oriented to the Pt (111) plane in Example 1 is stronglyoriented to KNN (001), as shown in FIG. 2.

In Comparative Example 2 in which the Pt thin film was formed at a roomtemperature without heating the substrate, it was confirmed that, as aresult of examining the Pt thin film by X-ray diffraction measurement,diffraction form the specific crystal surface does not exist and the Ptthin film is in an amorphous state. In addition, it was found thatorientation is not present in the KNN film formed on the Pt film inComparative Example 2 which was formed at a room temperature, and theKNN film was a random polycrystalline film.

Next, the surface irregularities on the piezoelectric layer 4 of the KNNfilms formed on the bottom electrode layer 3 having different surfaceirregularities in Example 1 and Comparative Example 1 was visuallyobserved, and it was confirmed that the KNN film formed on the Ptelectrode having large surface irregularities in Comparative Example 1has a white turbidity like opaque pattern on a surface thereof. On theother hand, the surface of the KNN film formed on the Pt electrodehaving small surface irregularities in Example 1 was in a clear mirrorsurface condition.

Furthermore, the surface irregularities of the two KNN films wereexamined by the AFM. Similarly to the Pt bottom electrode, the surfaceroughness was derived by measuring at 4 nm pitch in an area of 1 μm×1μm. FIG. 4A is an AFM observation image showing a surface of the KNNfilm in Comparative Example 1. In addition, FIG. 4B is an AFMobservation image showing a surface of the KNN film in Example 1.

From the above analysis result, as for the KNN film formed on the upperportion of the Pt electrode having large surface irregularities inComparative Example 1, the values of the arithmetic mean roughness Ra,the root mean square roughness Rms and the maximum-minimum roughnessRmax were respectively 21 nm, 28 nm and 199 nm. In contrast, as for theKNN film formed on the upper surface of the Pt electrode having smallsurface irregularities in Example 1, the arithmetic mean roughness Ra,the root mean square roughness Rms and the maximum-minimum roughnessRmax of the surface were respectively 4.0 nm, 3.2 nm and 28 nm.

From this analysis result, it was found that it is possible tosignificantly planarize a surface condition of the KNN film bycrystallizing the Pt bottom electrode so as to orient to a (111) plane,decreasing the surface irregularities of the Pt bottom electrode andforming the KNN film on an upper portion thereof.

In addition, composition of a sintered body sputtering target waschanged and examined, and it was found that the surface condition of thepiezoelectric thin film was significantly planarized in the same mannerwhen (Na_(x)K_(y)Li_(z))NbO₃ is in a range of 0≦x≦1, 0≦y≦1, 0≦z≦0.2 andx+y+z=1, and there were at least one or more portions which meet any ofa portion in which the arithmetic mean roughness Ra of the surface is4.1 nm or less, a portion in which the root mean square roughness Rms is4.8 nm or less and a portion in which the maximum-minimum roughness Rmaxis 137.7 nm or less. Furthermore, the result was the same in the casewhere Ta was doped to the above-mentioned target.

Example 2

FIG. 5 is a cross sectional view showing a piezoelectric thin filmelement in Example 2. In Example 2, a piezoelectric thin film elementwas made in which a bottom electrode layer 3, a base layer 6 as anorientation control layer for improving the orientation of the KNN film,a piezoelectric layer 4 of the KNN film and a top electrode 5 are formedon an upper portion of the bonding layer 2 which is formed on the Sisubstrate 1 having an oxide film.

Similarly to Example 1, the bottom electrode layer 3 of the Pt thin filmwas formed on the Si substrate 1 via the bonding layer 2 of the Ti film,and the bottom electrode layer 3 was crystallized, oriented to Pt (111)and planarized. Next, on the Pt bottom electrode layer 3, a LaNiO₃(lanthanum nickel oxide; LNO) film was formed as the base layer 6. TheLNO film is easily oriented to a (001) plane on the Pt (111) plane. TheLNO film was formed also using the sputtering method. A mixed gas ofAr+O₂ (a mixture ratio of Ar:O₂=9:1) was used as a sputtering gas. Thefilm is formed 200 nm thick at 75 W of RF power. When the X-raydiffraction measurement was carried out on the LNO film, it was foundthat the LNO film is independently oriented to LNO (001).

The piezoelectric layer 4 of the KNN film was formed on the base layer 6of the LNO film. The formation condition was the same as that ofExample 1. As a result of evaluating the oriented state of the thusformed KNN film by using an X-ray diffractometer, it was found that thethus formed KNN film is strongly oriented to KNN (001) than to thatformed in Example 1.

In addition, the surface irregularities of the KNN film were evaluatedusing the AFM. The AFM with a probe tip having a curvature radius of 1nm or more was used, and the surface roughness was calculated bymeasuring at 20 nm pitch in an area of 10 μm×10 μm. As a result, it wasfound that the arithmetic mean roughness Ra, the root mean squareroughness Rms and the maximum-minimum roughness Rmax were respectively3.0 nm, 3.8 nm and 46.8 nm and the mirror condition became good. Fromthis result, it was found that it is possible to planarize by orientingthe KNN film.

Next, the top electrode 5 was formed on the piezoelectric layer 4 of theformed KNN film. Al (aluminum) was selected for a material of the topelectrode 5 and a vacuum deposition method was used for the formation.The surface irregularities were evaluated also for the top electrode 5.As a result, it was found that the arithmetic mean roughness Ra, theroot mean square roughness Rms and the maximum-minimum roughness Rmax ofthe surface were respectively 3.0 nm, 3.6 nm and 40.2 nm and the topelectrode 5 was sufficiently flat. From this result, it was confirmedthat the surface of the top electrode 5 formed on the upper portion ofthe flat piezoelectric layer 4 has the substantially same flatness asthe piezoelectric layer 4.

Next, when composition of a sputtering target was changed and examined,it was found that the surface condition became a mirror surface in thesame manner when (Na_(x)K_(y)Li_(z))NbO₃ is in a range of 0≦x≦1, 0≦y≦1,0≦z≦0.2 and x+y+z=1, and there were at least one or more portions, whichmeet any of a portion in which the arithmetic mean roughness Ra of thesurface is 4.1 nm or less, a portion in which the root mean squareroughness Rms is 4.8 nm or less and a portion in which themaximum-minimum roughness Rmax is 137.7 nm or less.

Example 3

FIG. 6 is a cross sectional view showing a piezoelectric thin filmelement in Example 3. In Example 3, a piezoelectric thin film elementwas made in which a bottom electrode layer 3, a base layer 7 of sodiumniobate, a piezoelectric layer 8 of lithium potassium sodium niobate anda top electrode 5 are formed on an upper portion of the bonding layer 2which is formed on the Si substrate 1 having an oxide film.

In Example 3, an effect of using a base layer (an orientation controllayer) different from LaNiO₃ of the above-mentioned Example 2 wasexamined. Sodium niobate (NaNbO₃) was used as the base layer 7. Inaddition, in Example 3, lithium potassium sodium niobate((Na_(x)K_(y)Li_(z))NbO₃, herein referred to as “LKNN”), which islithium-doped KNN, was used as the piezoelectric layer 8. Although LKNNis composed of five elements, which are lithium (Li), potassium (K),sodium (Na), niobium (Nb) and oxygen (O), since inside of a chamber forperforming the sputtering could not be contaminated by a substance otherthan a component of the piezoelectric layer 8 if sodium niobate (NaNbO₃)not containing lithium or potassium is used as the base layer 7 amongthem, it is possible to form the piezoelectric layer 8 using the samechamber as that of the base layer 7, and it is thereby possible tocontinuously form the base layer 7 and the piezoelectric layer 8.

Firstly, a substrate with a Pt film which is the same as that of Example2 was prepared, and then, the base layer 7 of sodium niobate was formedon an upper portion of the bottom electrode layer 3. The sputteringmethod was used for forming the base layer 7. The base layer 7 wasformed 200 nm thick at 100 W of RF power by using an Ar+O₂ mixed gas (amixture ratio of Ar:O₂=8.5:1.5) as a sputtering gas. The sodium niobatefilm thus formed was evaluated by the X-ray diffractometer, and it wasfound that the film is preferentially oriented to a (001) plane.

Next, the piezoelectric layer 8 of the LKNN film was formed on the baselayer 7 of sodium niobate film. The sputtering method was used forforming the LKNN film. The substrate was heated at 600° C. during thefilm formation and the sputtering was performed by plasma using a mixedgas of Ar+O₂ (a mixture ratio of Ar:O₂=9:1). A sintered body target of(Na_(x)K_(y)Li_(z))NbO₃ (x=0.48, y=0.48 and z=0.04) was used for atarget. The film formation was performed at 100 W of RF output until thefilm thickness becomes 3 μm.

A crystalline property of the LKNN film thus formed was evaluated usingthe X-ray diffractometer, and it was found that the LKNN film isoriented to two planes, which are (110) and (001) planes. The surfaceirregularities of the specimen were evaluated using the AFM. For theevaluation, the surface roughness was calculated by measuring at 20 nmpitch in a range of 10 μm×10 μm. As a result, the arithmetic meanroughness Ra, the root mean square roughness Rms and the maximum-minimumroughness Rmax of the surface were respectively 3.9 nm, 6.4 nm and 137.7nm and the surface became a good mirror condition.

From the above results, it was found that, as long as a material can beformed on the Pt (111) electrode so as to be oriented, it is possible touse the material as a base layer. Some of such materials were furtherexamined. As a result, there was an effect when LaAlO₃, SrTiO₃, SrRuO₃,La_(0.06)Sr_(0.04)FeO₃, La_(0.06)Sr_(0.4)CoO₃ and KNbO₃ were usedbesides LaNiO₃ and NaNbO₃. In addition, there was an effect bylaminating (forming KNbO₃ on LaNiO₃, etc.) or solidifying (La(Ni, Al)O₃, etc.) thereof.

In addition, the top electrode 5 was formed on the piezoelectric layer 8of the above-mentioned KNN film. Al was selected for a material of thetop electrode 5 and a vacuum deposition method was used for theformation. The surface irregularities were evaluated also for the topelectrode 5. As a result, it was found that the arithmetic meanroughness Ra, the root mean square roughness Rms and the maximum-minimumroughness Rmax of the surface were respectively 3.0 nm, 5.5 nm and 110.3nm and the top electrode 5 was sufficiently flat. From this examination,it was confirmed that the surface of the top electrode formed on theupper portion of the flat piezoelectric layer has the substantially sameflatness as the piezoelectric layer.

Example 4

In Example 4, it was tried to manufacture the same structure as that ofthe KNN film on the Pt electrode oriented to (111) in Example 1 by usinga substrate other than a Si substrate. In Example 1, a Si substrate isused and a thermal oxide film is formed on an upper portion thereof.Since the thermal oxide film is in an amorphous state, the Pt bottomelectrode layer, which is formed on the upper portion of the thermaloxide film and is oriented to Pt (111), does not inherits the crystalstructure of the Si substrate. The Pt (111) orientation is a result ofself-orientation to a close-packed plane of Pt which is a cubicalcrystal. From this, it was considered that, regardless of the crystalstructure of the substrate, it is possible to form the same structureeven on a substrate other than the Si substrate, thus, the examinationwas carried out. As a result, it was confirmed that the same effect isobtained when the Pt electrode layer of the bottom electrode layer isdirectly formed on a silica glass substrate, a MgO substrate or a SrTiO₃substrate. In addition, it was found that the same effect is obtained byforming a Si oxide film on a glass substrate, a Ge substrate and a SUSsubstrate by PE-CVD (Plasma Enhanced Chemical Vapor Deposition; plasmaCVD) instead of forming a thermal oxide film in Example 1.

Relation Between Surface Roughness and Piezoelectric Constant

Next, an appropriate value (range) of the piezoelectric layer and thebottom electrode with respect to the effective piezoelectric constant inthe KNN piezoelectric thin film element was examined. FIG. 7 is acorrelation diagram between each piezoelectric constant and appliedelectric field of the piezoelectric element of the KNN film formed onthe Pt bottom electrode having different surface roughness (the rootmean square roughness Rms is shown here). As understood from FIG. 7, thepiezoelectric constants of the piezoelectric elements of both KNN filmsincrease with increasing electric field. In addition, what is clear isthat, when the surface roughness of the Pt bottom electrode is small(when the surface roughness is reduced), the piezoelectric constant isincreased. Particularly, at 6 MV/m of the applied electric field, thepiezoelectric constant is about 100 [arbitrary units] when Rms of the Ptbottom electrode is 1.1 nm (Example 1), and the piezoelectric constantis about 70 [arbitrary units] when Rms of the Pt bottom electrode is 2.5nm (Comparative Example 1).

Here, the reason why the piezoelectric constant is quantified byarbitrary units is as follows. Although a value of Young's modulus orPoisson's ratio, etc., of the piezoelectric layer is required forderiving the piezoelectric constant, it is difficult to derive the valueof Young's modulus or Poisson's ratio, etc., of the piezoelectric layer(piezoelectric thin film). Particularly, unlike a bulk body, since athin film is affected by a substrate used at the time of the filmformation (constraint, etc.), it is difficult to derive, in principle,an absolute value (true value) of the Young's modulus or the Poisson'sratio (constant) of the thin film itself. Then, the piezoelectricconstant is calculated using an estimated value of the Young's modulusor the Poisson's ratio of the KNN film which has been known so far.Therefore, since the obtained piezoelectric constant is an estimatevalue, the piezoelectric constant is relative arbitrary units in orderto be objective. In this regard, however, even though the value of theYoung's modulus or the Poisson's ratio of the KNN film used forcalculating the piezoelectric constant is an estimate value, it is areliable value in some degrees, and about 100 [arbitrary units] of thepiezoelectric constant can be appropriately 100 [-pm/V] of piezoelectricconstant d₃₁.

In addition, FIGS. 8A, 8B and 8C show correlation diagrams betweensurface roughness Rms, Ra and Rmax of the Pt bottom electrode and thepiezoelectric constant as one of Examples. Horizontal axes in FIGS. 8A,8B and 8C are the arithmetic mean surface roughness Ra, the root meansquare surface roughness Rms and the maximum-minimum roughness Rmaxderived by evaluating the surface irregularities using the AFM asmentioned earlier. The unit is nm. As shown in FIGS. 8A to 8C,regardless of magnitude of the applied electric field, the piezoelectricconstant increases with decreasing the surface roughness of the Ra, Rmsand Rmax. Particularly, at 6.7M of applied electric field, when thearithmetic mean surface roughness Ra becomes 5.3 nm or the root meansquare surface roughness Rms becomes 6.5 nm, the piezoelectric constantbecomes substantially zero. In FIGS. 8A to 8C, a region where thepiezoelectric constant becomes substantially zero is hatched by diagonallines.

Even if the used applied electric field is about 6.7 MV/m, as understoodfrom FIG. 8A, the surface roughness of the bottom electrode, by which apiezoelectric constant of the same level as the piezoelectric thin filmelement of the PZT thin film in a prior art such as over 100 [arbitraryunits] of the piezoelectric constant can be ensured, is 0.86 nm or lessin Ra or 1.1 nm or less in Rms.

Next, FIGS. 9A and 9B show correlation diagrams between the surfaceroughness Ra and Rms of the Pt bottom electrode and the surfaceroughness of the KNN piezoelectric thin film formed on an upper portionthereof. As understood from the figures, the surface roughness of theKNN film is 21 nm when the surface roughness Ra of the Pt bottomelectrode is 2.3 nm, however, when the surface roughness Ra of the Ptbottom electrode is small such as 0.68 nm, the surface roughness of theKNN film is 2.0 nm. It is understood that the surface roughness of theKNN thin film increases with increasing the surface roughness of the Ptbottom electrode. In other words, it is possible to decrease the surfaceroughness of the piezoelectric thin film by reducing the surfaceroughness of the bottom electrode.

FIGS. 10A and 10B show correlation diagrams between arithmetic meansurface roughness Ra and the root mean square surface roughness Rms ofthe KNN thin film and the piezoelectric constant as one of Examples. Asshown in the figures, regardless of magnitude of the applied electricfield, the piezoelectric constant increases with decreasing the surfaceroughness of the KNN thin film. Conversely, the piezoelectric constantdecreases with increasing the surface roughness Rms and Ra. It ispossible to manufacture a piezoelectric element with performancerequired for various devices by appropriately controlling the surfaceroughness of the Pt bottom electrode layer and the KNN thin film. Inaddition, it is necessary to increase or decrease the film thicknessdepending on devices from a viewpoint of controlling an elementalstrength, etc.

FIGS. 11A and 11B are correlation diagrams between a relative value(unit: %) of the arithmetic mean roughness Ra and the root mean squareroughness Rms with respect to an average film thickness of the KNN thinfilm and a piezoelectric constant of the KNN piezoelectric thin filmelement. At 6.7 MV/m of applied electric field, when the relative valueof the arithmetic mean surface roughness Ra with respect to the filmthickness becomes about 2.3% or the relative value of the root meansquare surface roughness Rms becomes about 3.1%, the piezoelectricconstant becomes substantially zero. In other words, if it is consideredthat the mainly used applied electric field is about 7 MV/m, the averagesurface roughness Ra and Rms with respect to the film thickness of theKNN thin film, by which a desired piezoelectric constant in theinvention can be ensured, is about 2.7% or less.

Furthermore, FIG. 12 is a correlation diagram between a relative value(unit: %) of maximum-minimum roughness Rmax with respect to the averagefilm thickness of the KNN thin film and a piezoelectric constant of theKNN piezoelectric thin film element. As shown in the figures, regardlessof magnitude of the applied electric field, the piezoelectric constantincreases with decreasing the relative surface roughness of the KNN thinfilm. When it is considered that the used applied electric field isabout 7 My/m, the relative surface roughness of the KNN thin film, bywhich a desired piezoelectric constant in the invention can be ensured,is about 23% or less. In FIG. 12, a region where the piezoelectricconstant becomes zero is hatched by diagonal lines.

Although the invention has been described with respect to the specificembodiment for complete and clear disclosure, the appended claims arenot to be therefore limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A piezoelectric thin film element, comprising: a bottom electrode; apiezoelectric layer; and a top electrode on a substrate, wherein thepiezoelectric layer comprises, as a main phase, a perovskite-type oxide,wherein the bottom electrode has a surface roughness of not more than0.86 nm in arithmetic mean roughness Ra or not more than 1.1 nm in rootmean square roughness Rms, and wherein the bottom electrode has a (111)preferential orientation in a direction perpendicular to the substrate.2. The piezoelectric thin film element according to claim 1, wherein thepiezoelectric layer formed on the bottom electrode has a maximum-minimumsurface roughness of not more than 23% with respect to an average filmthickness of the piezoelectric layer.
 3. The piezoelectric thin filmelement according to claim 1, wherein the piezoelectric layer formed onthe bottom electrode has the arithmetic mean surface roughness Ra or theroot mean square surface roughness Rms of not more than 2.7% withrespect to an average film thickness of the piezoelectric layer.
 4. Thepiezoelectric thin film element according to claim 1, wherein thepiezoelectric layer has the surface roughness of not more than 4.1 nm inarithmetic mean roughness Ra, not more than 4.8 nm in the root meansquare roughness Rms, or not more than 137.7 nm in a maximum-minimumroughness.
 5. The piezoelectric thin film element according to claim 1,wherein the bottom electrode has the surface roughness of not more than1.1 nm in the root mean square roughness Rms.
 6. A piezoelectricactuator, comprising: a piezoelectric thin film element according toclaim 1; and a voltage application unit which applies a voltage to thepiezoelectric thin film element.
 7. A piezoelectric sensor, comprising:a piezoelectric thin film element according to claim 1; and a voltagedetecting unit which detects a voltage generated at the piezoelectricthin film element.